Dehumidifying systems to produce a controlled humidity environment in an enclosed volume have found applications in industrial processing, product drying, material storage and comfort air conditioning. Typical apparatus and methods which are employed in such systems involve a physical adsorption process in which moisture is removed from air by the use of a desiccant, such as a silica gel or other hydrophylic material.
When a desiccant is utilized, ambient air is directed through a desiccant bed in which the desiccant medium adsorbs moisture (water vapor), typically up to 40% of the weight of the desiccant, depending upon the desiccant employed. After a predetermined time period, the adsorption limit for the desiccant approaches a design limit, and the desiccant is reactivated by the application of heated air to dry the bed. After reactivation, the same desiccant is reused for adsorption. The process of adsorption by, and reactivation of, the desiccant is cyclically repeated.
One type of commercially available dehumidifier includes a circular desiccant filled bed in which a thickness of desiccant is covered on each side with circular perforated sheet steel disks to permit controlled air passage through the bed in which one section is utilized for drying process air and another section is used for reactivation of the desiccant after the predetermined time limit of a cycle is reached. Such circular beds are enclosed in a manner in which sections of the same bed are compartmentally divided by an enclosure. The circular desiccant beds are rotated continuously through the compartments which are isolated from each other by dividing means which generally use silastic or other pliable seals in contact with the bed. One compartment, accessible to a principal segment of the bed, dehumidifies the process air and delivers a continuous, uninterrupted supply of dry air to the area or process requiring it. In the other compartment, a portion of the desiccant bed is regenerated by an application of heated air and made ready for reuse. Thus, the desiccant in the circular bed is continuously used, and then reactivated, as the thick, circular bed rotates to present different portions of the bed, respectively to the compartments of the enclosure intended for process air and the reactivating stage.
A problem encountered in a circular bed system is that sections of the desiccant bed cannot be absolutely isolated with respect to a section in process use and a section subjected to heated reactivating air flow. Further, co-mingling of currents of air being conditioned and the heated reactivating air may occur in the bed or at the compartmental seal between bed sections--resulting in mixture of the two air flows. This occurs because the conventional design requires co-current air flow (i.e., in the same direction) of the process and reactivating air stream or because of incidental or design imbalances between the two air streams. In addition, in a circular bed dryer, the sections are not separated and air moves horizontally through the bed sections. Also, pliable seals which separate the compartments with respect to the bed are subjected to contact with the circular perforated surface of the bed enclosure disks and abrasion of the seal may result. In addition, the desiccant itself may leach through the perforated disks of the bed, causing further abrasion problems with respect to the seal gasket and the circular bed movement with respect thereto.
An inherent inefficiency in a co-current adsorber system is that, depending upon the thickness of the desiccant bed, moisture may tend to concentrate on the one side of the bed which is first subjected to process air flow as the air is directed through the thickness of the desiccant within the bed. In the reactivation stage in a co-current system, the concentrated moisture on the one side must also be "driven" through the thickness of the bed desiccant before complete drying occurs. Similarly, heat of reactivation also tends to concentrate on the one side of the bed, and in cooling the bed after reactivation, before the desiccant reaches an optimum efficiency, heat concentrated on one side after bed, similarly is driven through the bed thickness. The degree of inefficiency encountered in driving moisture through the bed thickness in the drying stage and in driving heat through the bed in the cooling stage is, of course, dependent upon operating parameters such as bed thickness, conductivity, air flow rates and the like which will be evident to those of skill in the art; however, as the size and capacity of a desiccant bed dehumidifying unit increases, these factors begin more importantly to affect operating efficiency and cost.
In addition, various drum and/or chambered apparatus are also known to be useful in air drying dehumidifying units. For example, U.S. Pat. No. 3,487,608, Roderick W. Graff, "Method and Apparatus for Adsorption of Molecules from Fluids," issued Jan. 6, 1970, describes an adsorption apparatus using multiple separate chambers filled with adsorbent material, which are cyclically utilized for adsorption and then subjected to regeneration. The chambers may be rotatably disposed to provide respective operative engagement to an adsorbing function and a regenerating function in a cyclical manner between different chambers. Also, a rotatable drum has also been employed in an adsorbent system in which an annular drum is divided by partitions into a number of compartments which are cyclically subjected to processing and regeneration. See: U.S. Pat. No. 3,639,000 W. E. Edwards, "Rotating Bed Absorber," issued May 19, 1953.